wave generator used to produce a narrow pulse, or trigger. Blocking oscillators have many uses, most of which are concerned with the timing of some other circuit. They can be used as frequency dividers or counter circuits and for switching other circuits on and off at specific times.">

The BLOCKING OSCILLATOR is a special type of wave generator used to produce a narrow
pulse, or trigger. Blocking oscillators have many uses, most of which are concerned with
the timing of some other circuit. They can be used as frequency dividers or counter
circuits and for switching other circuits on and off at specific times.

In a blocking oscillator the pulse width (pw), pulse repetition time
(prt), and pulse
repetition rate (prr) are all controlled by the size of certain capacitors and resistors
and by the operating characteristics of the transformer. The transformer primary
determines the duration and shape of the output. Because of their importance in the
circuit, transformer action and series RL circuits will be discussed briefly. You may want
to review transformer action in NEETS, Module 2, Introduction to Alternating Current
and Transformers before going to the next section.

Transformer Action

Figure 3-31 , view (A), shows a transformer with resistance in both the primary and
secondary circuits. If S1 is closed, current will flow through R1 and L1. As the current
increases in L1, it induces a voltage into L2 and causes current flow through R2. The
voltage induced into L2 depends on the ratio of turns between L1 and L2 as well as the
current flow through L1.

Figure 3-31A. - RL circuit.

The secondary load impedance, R2, affects the primary impedance through reflection from
secondary to primary. If the load on the secondary is increased (R2 decreased), the load
on the primary is also increased and primary and secondary currents are increased.

T1 can be shown as an inductor and R1-R2 as a combined or equivalent series resistance
(RE) since T1 has an effective inductance and any change in R1 or R2 will
change the current. The equivalent circuit is shown in figure 3-31, view (B). It acts as a
series RL circuit and will be discussed in those terms.

Figure 3-31B. - RL circuit.

Simple Series RL Circuit

When S1 is closed in the series RL circuit (view (B) of figure 3-31) L acts as an open
at the first instant as source voltage appears across it. As current begins to flow, EL
decreases and ER and I increase, all at exponential rates. Figure 3-32, view
(A), shows these exponential curves. In a time equal to 5 time constants the resistor
voltage and current are maximum and EL is zero. This relationship is shown in
the following formula:

Figure 3-32A. - Voltage across a coil.

If S1 is closed, as shown in figure 3-31, view (B), the current will follow curve 1 as
shown in figure 3-32, view (A). The time required for the current to reach maximum depends
on the size of L and RE. If RE is small, then the RL circuit has a
long time constant. If only a small portion of curve 1 (C to D of view (A)) is used, then
the current increase will have maximum change in a given time period. Further, the smaller
the time increment the more nearly linear is the current rise. A constant current increase
through the coil is a key factor in a blocking oscillator.

Blocking Oscillator Applications

A basic principle of inductance is that if the increase of current through a coil is
linear; that is, the rate of current increase is constant with respect to time, then the
induced voltage will be constant. This is true in both the primary and secondary of a
transformer. Figure 3-32, view (B), shows the voltage waveform across the coil when the
current through it increases at a constant rate. Notice that this waveform is similar in
shape to the trigger pulse shown earlier in figure 3-1, view (E). By definition, a
blocking oscillator is a special type of oscillator which uses inductive regenerative
feedback. The output duration and frequency of such pulses are determined by the
characteristics of a transformer and its relationship to the circuit. Figure 3-33 shows a
blocking oscillator. This is a simplified form used to illustrate circuit operation.

Figure 3-32B. - Voltage across a coil.

Figure 3-33. - Blocking oscillator.

When power is applied to the circuit, R1 provides forward bias and transistor Q1
conducts. Current flow through Q1 and the primary of T1 induces a voltage in L2. The
phasing dots on the transformer indicate a 180-degree phase shift. As the bottom side of
L1 is going negative, the bottom side of L2 is going positive. The positive voltage of L2
is coupled to the base of the transistor through C1, and Q1 conducts more. This provides
more collector current and more current through L1. This action is regenerative feedback.
Very rapidly, sufficient voltage is applied to saturate the base of Q1. Once the base
becomes saturated, it loses control over collector current. The circuit now can be
compared to a small resistor (Q1) in series with a relatively large inductor (L1), or a
series RL circuit.

The operation of the circuit to this point has generated a very steep leading edge for
the output pulse. Figure 3-34 shows the idealized collector and base waveforms. Once the
base of Q1 (figure 3-33) becomes saturated, the current increase in L1 is determined by
the time constant of L1 and the total series resistance. From T0 to T1 in figure 3-34 the
current increase (not shown) is approximately linear. The voltage across L1 will be a
constant value as long as the current increase through L1 is linear.

Figure 3-34. - Blocking oscillator idealized waveforms.

At time T1, L1 saturates. At this time, there is no further change in magnetic flux and
no coupling from L1 to L2. C1, which has charged during time TO to T1, will now discharge
through R1 and cut off Q1. This causes collector current to stop, and the voltage across
L1 returns to 0.

The length of time between T0 and T1 (and T2 to T3 in the next cycle) is the pulse
width, which depends mainly on the characteristics of the transformer and the point at
which the transformer saturates. A transformer is chosen that will saturate at about 10
percent of the total circuit current. This ensures that the current increase is nearly
linear. The transformer controls the pulse width because it controls the slope of
collector current increase between points T0 and T1. Since TC = L / R , the greater the L,
the longer the TC. The longer the time constant, the slower the rate of current increase.
When the rate of current increase is slow, the voltage across L1 is constant for a longer
time. This primarily determines the pulse width.

From T1 to T2 (figure 3-34), transistor Q1 is held at cutoff by C1 discharging through
R1 (figure 3-33). The transistor is now said to be "blocked." As C1 gradually
loses its charge, the voltage on the base of Q1 returns to a forward-bias condition. At
T2, the voltage on the base has become sufficiently positive to forward bias Q1, and the
cycle is repeated.